Attrition resistance is an important aspect of fluidizable particles, such as catalysts, sorbent materials, reaction surface supports and the like. Fluidizable particles in fluidized beds are used in numerous chemical conversion applications, including catalytic conversions, absorption reactions, and the like. Numerous techniques have been developed over the years to improve attrition resistance of such particles. However, in many cases, these techniques are not suitable for the particular materials used to form particles for a given application. In other cases, many of these techniques can be limited to use with particular particle sizes.
Fluidizable materials based on active zinc compounds sorbent particles have recently been developed for use in the desulfurization of coal gas. Coal represents our largest fossil energy source. The efficiency of converting the chemical energy stored in coal to electricity can be improved by first generating fuel gas via coal gasification, and then oxidizing the hot gas in either a turbine or a fuel cell. This approach, however, is complicated by the presence of sulfur in coal, which is converted to reduced sulfur species such as H.sub.2 S, COS, and CS.sub.2 during gasification. Subsequently, during combustion of the fuel gas, the H.sub.2 S oxidizes to SO.sub.2 which is environmentally undesirable. In addition to environmental concerns, high concentrations of H.sub.2 S can be corrosive to energy producing equipment and can adversely affect the performance of molten carbonate fuel cells due to sulfur poisoning of electrodes.
Early sorbent technology for sulfur removal from fuel gas was directed toward iron oxide. For example, U.S. Pat. No. 4,089,809 assigned to W. L. Farrior, disclosed a solid absorbent consisting of iron oxide supported on silica for removal of H.sub.2 S from hot gaseous mixtures. The efficacy of iron oxide sorbents for the absorption of H.sub.2 S is dictated by chemical equilibrium constraints. For example, at 550.degree. C. with 20 percent water vapor in coal gas, theoretically iron oxide sorbents can reduce H.sub.2 S level to about 361 ppm, but typically not any lower.
In contrast, studies performed using zinc oxide as a sorbent indicate that H.sub.2 S levels can be reduced to a few ppm. An example of a zinc oxide sorbent is proposed in U.S. Pat. No. 4,088,736 to Institut Francais du Petrole of France, which discusses zinc oxide sorbent supported on silica and/or alumina. Zinc oxide sorbents hailed as effective sorbents for reduced sulfur species; however, the actual gasifier gases are quite reducing, and zinc oxide sorbents are typically not as effective in these environments as is required in the industry. Pure zinc oxide sorbents are known to lose zinc due to reduction by carbon monoxide and/or hydrogen, both of which are present in fuel gas.
Subsequently, zinc ferrite sorbents were developed. U.S. Pat. No. 4,769,045 to Grindley proposes a representative zinc ferrite sorbent prepared from mixing and calcination of equimolar amounts of zinc oxide and iron oxide. Zinc ferrite sorbents were employed in fixed-bed, moving-bed, and fluidized-bed reactors for desulfurization, but the concerns with respect to sorbent degradation in terms of loss in percent sulfur absorption capacity and mechanical strength remained.
Zinc titanate sorbents were initially developed as an answer to the need for a desulfurization sorbent which exhibits resistance to degradation at the high temperatures and highly reducing coal gas environments of the hot-gases. The use of zinc titanate sorbents as high temperature desulfurization sorbents is proposed in U.S. Pat. Nos. 4,313,820 and 4,725,415, both assigned to Phillips Petroleum Company. The sorbents proposed therein are discussed in relation to fixed bed applications, and have a particle size of 20 to 40 mesh, a size which is unsuitable for fluidized bed applications. U.S. Pat. No. 4,977,123 to Flytzani-Stephanopolous et al., proposes a method of making mixed metal oxide sorbents suitable for use in fixed bed reactors as well. The proposed mixed metal oxide absorbents are prepared using calcined powders of a desired composition as starting materials, adding water to form a paste, extruding the paste, and drying and heating the extruded paste to yield the desired extrudate strength. The oxides may be oxide mixtures of various metals such as for example, copper, iron, aluminum, zinc, titanium, and mixtures thereof. Inorganic binder materials such as bentonite clay may also be added. The proposed method involves a series of complex, hard-to-reproduce, and potentially expensive steps which discourage its use in commercial applications.
Although fluidized bed reactors provide a particularly efficient environment for removal of sulfur compounds from feed streams, zinc titanate sorbent materials suitable for fluidized bed reactor applications have eluded investigators due to the required particle size range, typically from 40 to 300 microns. Fluidized bed reactor applications also require sorbents exhibiting good absorption rate and capacity for sulfur compounds, good regenerability without appreciable loss of efficacy or efficiency, and high attrition resistance.
Attempts have been made to improve the attrition resistance of zinc titanate materials for fluidized bed applications. U.S. Pat. No. 4,477,592 to Aldag, Jr. proposes a process for making a zinc titanate sorbent which includes a hydrogelling step designed to improve attrition resistance. The proposed hydrogelling step, involves dispersion of a finely powdered zinc titanate in a suspension of .alpha.-alumina monohydrate with the addition of nitric acid to form a hydrosol, which is dried, calcined at 648.degree. C. for 2 hours, and crushed and screened to produce a 420 to 1190 micron particle size catalyst for use in a transport reactor. The hydrogelling step improved attrition resistance at the expense of reduced catalytic activity primarily due to decreased zinc titanate content.
U.S. Pat. No. 5,254,516, issued Oct. 19, 1993 to Gupta et al discloses highly durable and chemically reactive zinc titanate sorbents having a particle size range of between 50 and 400 microns which are prepared by granulating a mixture of fine zinc oxide and titanium dioxide with an inorganic binder, typically bentonite and/or kaolinite, and an organic binder, and then indurating the granules. The resultant sorbent particles have high particle density and are highly attrition resistant and advantageously are capable of absorbing significant quantities of sulfur compounds from a feed stream. Because of their size, reactivity and durability, these sorbent materials are suitable for use in fluidized bed reactors. However, the granulation process results in the production of particles having sizes distributed over a relatively wide range and typically a substantial portion of the product, e.g., 30-40 wt. percent, has a size too large for use in a fluidized-bed reactor and must be discarded. A related publication describes work leading to the development of this sorbent material; see R. P. Gupta and S. K. Gangwal, "Enhanced Durability of Desulfurization Sorbents for Fluidized Bed Applications", Topical Report to DOE/METC, Contract No. DE-AC21-88MC25006, November, 1992, NTIS No. NTIS/DE93000247. As detailed therein, numerous particle-forming techniques and zinc titanate/binder compositions were investigated prior to the development of these attrition-resistant zinc titanate sorbents.
One particularly desirable technique for the production of a particulate sorbent or catalyst suitable for use in fluidized-beds is the spray drying process. This process has been employed extensively in the production of various catalysts, particularly fluid cracking catalysts. Spray drying offers a number of advantages over granulation or agglomeration particulate-forming processes. For example, spray drying is a commercial process which can be readily scaled to commercial production using existing technology to produce large quantities of a product. Spray drying facilitates the addition of other additives and reagents to the composition since additional reagents can simply be added to a slurry prior to spray drying. Spray drying can also provide particles of highly uniform size and shape. In the production of fluidized bed catalysts, the uniformity of the particulate product results in improved process economics in the form of a higher product yield. In many cases, conventional spray drying techniques can provide nearly a 100 percent yield of particles having a size suitable for use in a fluidized bed. In such cases, little, if any, of the spray dried particles must be discarded as waste.
Spray drying processes are well known in the art and are disclosed in numerous publications. In one spray drying process disclosed in U.S. Pat. No. 4,946,814 to Shi et al., an acid stable surfactant is used in combination with a silica-sol binder system to provide FCC catalysts of significantly improved morphology, selectivity, and attrition resistance. The acid stable surfactant can be added to any one, or all, of the final slurry components including the alumina-silica sol slurry, the clay slurry, the alumina and/or the zeolite slurry.
Conventional spray drying involves the use of silica binders to produce coherent, attrition resistant particles. Although spray dried zinc titanate sorbents formed by conventional spray drying techniques and using conventional binders do exhibit improved attrition resistance, this has been achieved only at the expense of sulfur absorption capacity. Various attempts to prepare spray dried zinc titanate catalysts were made during the study described in the previously mentioned November, 1992, publication of R. P. Gupta and S. K. Gangwal, NTIS No. NTIS/DE930000247. In summary, when either a colloidal silica binder, or a silica binder in the form of polysilicic acid, were employed in the formulations, reactivity of the final sorbent was substantially eliminated. In one portion of the study a skilled outside contractor was commissioned to prepare a group of spray dried zinc titanate sorbents including zinc titanate sorbents free of silica and comprising binders prepared from only bentonite and an organic binder. However the resultant sorbents exhibited poor attrition resistance and an unacceptably low sulfur absorption capacity of less than 12 weight percent sulfur absorption. Subsequent attempts to produce spray dried zinc titanate sorbent materials during this work resulted in the failure to produce a stable slurry, resulting in turn, in failure of the particulate-forming spray drying step.
More recently in work related to the present invention, and disclosed in U.S. Pat. No. 5,714,431, issued Feb. 3, 1998 to Gupta et al., highly uniform and attrition zinc titanate particulate materials of high reactively have been prepared. These materials are spherical particles of uniform size and high reactivity. However, when the composition of the slurry was adjusted sufficiently to increase the average size of the particles, the final particles exhibited substantially decreased attrition resistance.
Spray dried fluidizable catalyst and sorbent materials of high porosity and attrition resistance and an increased average particle size, e.g., above about 80-100 micron, are also desirable in a variety of other chemical treatment and conversion processes throughout the art. However, conventional techniques involving a spray drying step are generally unsuccessful in providing such materials.